Scotland captain Stuart McInally is an injury doubt for the summer tour after suffering a calf strain.
Scottish Rugby says the hooker, who was set to be captain for the first time, will remain with the squad.
Grant Stewart, 23, who has made seven appearances for Glasgow Warriors this season as well as representing Glasgow Hawks, has been called up as precautionary injury cover.
Scotland face Canada, the USA and Argentina next month.
Toulouse lock Richie Gray has already pulled out of the squad because of a hip injury.
Scotland will play on three consecutive Saturdays, starting with the game against Canada in Edmonton on 9 June.
They then play the United States in Houston a week later and the final Test is against Argentina in Resistencia on 23 June.